Square piles are widely utilized in coastal engineering due to their economic efficiency and robustness in resisting large forces in the coastal environment. However, the removal of sediment particles due to the approaching flow around such structures, known as scour, raises concerns about the stability and safety of the structure. Therefore, this study investigates scour around square piles placed at 45 deg and 90 deg angles in wave–current flows. A newly developed sediment transport module within the open-source REEF3D framework is developed, incorporating a three-phase semicoupled approach with level-set method (LSM) for realistic representation of sediment bed and free surface interfaces. The developed model is first validated against experimental results of circular and square pile scour in different flow conditions, such as steady current, wave-only, and wave–current flows. Furthermore, the effect of the combined wave–current parameter (U_cw) and Keulegan–Carpenter (KC) number on the normalized equilibrium scour depth (S/D_w) is explored. This study provides new insights into how square pile orientation modifies bed topography and equilibrium scour depth in wave–current flows. Numerical results demonstrate that a higher S/D_w value was observed for larger U_cw and KC numbers for both piles. It is revealed that in wave-only and combined wave–current flows with low KC numbers (KC < 10), square piles oriented at 45 deg experience greater scour depths than those oriented at 90 deg. However, at a higher KC number (KC = 18), square piles oriented at 90 deg exhibit greater scour depths compared to those at 45 deg.

Recent advancements have demonstrated that collectors based on the Coandă effect can effectively harvest polymetallic nodules from the seabed. However, the hydrodynamics of the flow around such collectors, particularly the mechanisms of ambient water entrainment, remain insufficiently explored. To address this gap, we performed three-dimensional numerical simulations to investigate the flow characteristics surrounding a Coandă effect-based collector, focusing on the effects of main jet velocity, secondary jet velocity, radius of curvature, and bottom clearance. The results show that increasing the main jet velocity enhances flow attachment and strengthens the pressure gradients beneath the collector, thereby increasing the entrainment of ambient water into the collection duct. Similarly, higher secondary jet velocities improve flow attachment and raise the collection duct flow rate but also lead to greater sideways water spillage. Furthermore, a larger radius of curvature reduces sideways spillage, consequently promoting greater ambient water entrainment beneath the collector. Likewise, increasing the bottom clearance enhances ambient water entrainment. Overall, these findings provide valuable insights for optimizing the operational parameters of Coandă effect-based collectors to maximize collection efficiency while minimizing water spillage.